This invention relates to a rotary pump having an end plate that is moved out of engagement with vanes therein to provide an unrestricted flow path between an inlet port and an outlet port whenever the fluid pressure differential reaches a predetermined value to thereafter reduce the work required to move the vanes by an input member.
In known pumps, the only way of reducing the work required to rotate the vanes is to disengage the input member from the power source through some type of clutch arrangement. Unfortunately when the power source is continually operating, noise can be created during the engagement of the clutch. In addition the cost of such clutches can limit the application of the pump.
With an increasing awareness of fuel efficiency it is estimated that up to 25% of the vehicles manufactured in 1985 will be powered by diesel engines. In order to provide continuity between the accessories used with internal combustion and diesel engines it will be necessary to provide a source of vacuum to operate many of the accessories. It has been determined that a continually operating pump can reduce the fuel efficiency of a diesel engine by about 5%. Since such a pump must be sized to meet peak demand of the accessories, during normal operation of the vehicle the demand for vacuum could be non-existent once the reserve capacity of vacuum is met. Thus, for optimum fuel efficiency, it is inoperative that the input force driving the pump be reduced once the operational demand for vacuum is achieved.